How to align object well enough to integrate into array modifier?
Joining your objects changed the plane's bounds, using the corridor's bounds. Array modifier is using the object's bounds as a reference (1) for the relative offset. The bounding box can be visible, it's a box containing your complete object.
  

Your plane is smaller than the bounding box, so it's repeated inside it. You need to fix it's topology to fit the bounds of your corridor.
  

To do this, activate vertex or edge snapping
  

Move the needed vertices/edges along 1 axis (Y on my sreenshot) with G, and  X, Y or Z. The mouse cusror will change when close to a vertex/edge it can use to snap
  

Do the same on the other side
  

Now your plane will fit the corridor's array.